This application exploits the cellular specificity of a simple behavioral system, the gill-withdrawal reflex of Aplysia, to study structural synaptic plasticity. Longterm memory for sensitization of this reflex is associated with the growth of new synaptic connections between identified sensory neurons and their follower cells. Similar structural changes can be reconstituted in sensory-motor neuron co-cultures by repeated presentations of serotonin. When a single bifurcated sensory neuron makes synapses with two spatially separated motor neurons, repeated application of serotonin to one set of synapses produces a CREB-mediated synapse-specific long-term facilitation which can be captured at the opposite synapse by a single serotonin pulse. Both of these processes are accompanied by persistent growth of new synaptic connections. When repeated serotonin pulses are applied to the cell body of the sensory neuron, they produce a cell-wide facilitation which is not associated with growth and does not persist, but that can be captured by a marking signal that stimulates growth and converts the transient facilitation to a persistent one. This project proposes to investigate the molecular mechanisms underlying the synaptic remodeling associated with specific forms of long-term facilitation. The aims of the experiments are (1) to determine the nature of the marking event required for the structural change, (2) to dissociate functional plasticity from the growth of new synaptic connections, (3) to examine the activity-dependence and the storage of synapse-specific long-term memory, and (4) to investigate the molecular mechanisms underlying learning-related synapse formation. It is expected that the principles derived from this reductionist approach might be applicable to more complex systems, and ultimately to human memory.